The following is a general description of art relevant to the present invention. None is admitted to be prior art to the invention. Generally, this art relates to observations relating to cellular senescence, and theories or hypotheses which explain such aging and the mechanisms by which cells escape senescence and immortalize.
Normal human somatic cells (e.g., fibroblasts, endothelial, and epithelial cells) display a finite replicative capacity of 50-100 population doublings characterized by a cessation of proliferation in spite of the presence of adequate growth factors. This cessation of replication in vitro is variously referred to as cellular senescence or cellular aging, See, Goldstein, 249 Science 1129, 1990; Hayflick and Moorehead, 25 Exp. Cell Res. 585, 1961; Hayflick, ibid., 37:614, 1985; Ohno, 11 Mech. Aging Dev. 179, 1979; Ham and McKeehan, (1979) "Media and Growth Requirements", W. B. Jacoby and I. M. Pastan (eds), in: Methods in Enzymology, Academic Press, N.Y., 58:44-93. The replicative life span of cells is inversely proportional to the in vivo age of the donor (Martin et al., 23 Lab. Invest. 86, 1979; Goldstein et al., 64 Proc. Natl. Acad. Sci. USA 155, 1969; and Schneider and Mitsui, ibid., 73:3584, 1976), therefore cellular senescence is suggested to play an important role in aging in vivo.
Cellular immortalization (the acquisition of unlimited replicative capacity) may be thought of as an abnormal escape from cellular senescence, Shay et al., 196 Exp. Cell Res. 33, 1991. Normal human somatic cells appear to be mortal, i.e., have finite replicative potential. In contrast, the germ line and malignant tumor cells are immortal (have indefinite proliferative potential). Human cells cultured in vitro appear to require the aid of transforming viral oncoproteins to become immortal and even then the frequency of immortalization is 10.sup.-6 to 10.sup.-7. Shay and Wright, 184 Exp. Cell Res. 109, 1989. A variety of hypotheses have been advanced over the years to explain the causes of cellular senescence. While examples of such hypotheses are provided below, there appears to be no consensus or universally accepted hypothesis.
For example, the free radical theory of aging suggests that free radical-mediated damage to DNA and other macromolecules is causative in critical loss of cell function (Harman, 11 J. Gerontol. 298, 1956; Harman, 16 J. Gerontol. 247, 1961). Harman says (Harman, 78 Proc. Natl. Acad. Sci. 7124, 1981) "aging is largely due to free radical reaction damage . . . "
Waste-product accumulation theories propose that the progressive accumulation of pigmented inclusion bodies (frequently referred to as lipofuscin) in aging cells gradually interferes with normal cell function (Strehler, 1 Adv. Geront. Res. 343, 1964; Bourne, 40 Prog. Brain Res. 187, 1973; Hayflick, 20 Exp. Gerontol. 145, 1985).
The somatic mutation theories propose that the progressive accumulation of genetic damage to somatic cells by radiation and other means impairs cell function and that without the genetic recombination that occurs, for instance, during meiosis in the germ line cells, somatic cells lack the ability to proliferate indefinitely (Burnet, "Intrinsic Mutagenesis--A Genetic Approach to Aging", Wile, N.Y., 1976; Hayflick, 27 Exp. Gerontol. 363, 1992). Theories concerning genetically programmed senescence suggest that the expression of senescent-specific genes actively inhibit cell proliferation (Martin et al., 74 Am. J. Pathol. 137, 1974; Goldstein, 249 Science 1129, 1990).
Smith and Whitney, 207 Science 82, 1980, discuss a mechanism for cellular aging and state that their data is:
"compatible with the process of genetically controlled terminal differentiation. . . . The gradual decrease in proliferation potential would also be compatible with a continuous build up of damage or errors, a process that has been theorized. However, the wide variability in doubling potentials, especially in mitotic pairs, suggests an unequalled partitioning of damage or errors at division." PA0 "informative oligonucleotide, built into DNA after a telogene and controlling synthesis of a repressor of differentiation, might serve as a means of counting mitosis performed in the course of morphogenesis. Marginotomic elimination of such an oligonucleotide would present an appropriate signal for the beginning of further differentiation. Lengthening of the telogene would increase the number of possible mitoses in differentiation." PA0 "Tumour cells are also characterized by shortened telomeres and increased frequency of aneuploidy, including telomeric associations. If loss of telomeric DNA ultimately causes cell-cycle arrest in normal cells, the final steps in this process may be blocked in immortalized cells. Whereas normal cells with relatively long telomeres and a senescent phenotype may contain little or no telomerase activity, tumour cells with short telomeres may have significant telomerase activity. Telomerase may therefore be an effective target for anti-tumour drugs. . . . PA0 There are a number of possible mechanisms for loss of telomeric DNA during ageing, including incomplete replication, degradation of termini (specific or nonspecific), and unequal recombination coupled to selection of cells with shorter telomeres. Two features of our data are relevant to this question. First, the decrease in mean telomere length is about 50 bp per mean population doubling and, second, the distribution does not change substantially with growth state or cell arrest. These data are most easily explained by incomplete copying of the template strands at their 3' termini. But the absence of detailed information about the mode of replication or degree of recombination at telomeres means that none of these mechanisms can be ruled out. Further research is required to determine the mechanism of telomere shortening in human fibroblasts and its significance to cellular senescence." [Citations omitted.] PA0 "[T]here is a reduction in the length of telomere repeat arrays relative to the normal colonic mucosa from the same patient. . . . PA0 Firm figures are not available, but it is likely that the tissues of a developed fetus result from 20-50 cell divisions, whereas several hundred or thousands of divisions have produced the colonic mucosa and blood cells of 60-year old individuals. Thus the degree of telomere reduction is more or less proportional to the number of cell divisions. It has been shown that the ends of Drosophila chromosomes without normal telomeres reduce in size by .sub.-- 4 base pairs (bp) per cell division and that the ends of yeast chromosomes reduce by a similar degree in a mutant presumed to lack telomerase function. If we assume the same rate of reduction is occurring during somatic division in human tissues, then a reduction in TRA by 14 kb would mean that 3,500 ancestral cell divisions lead to the production of cells in the blood of a 60-year old individual; using estimates of sperm telomere length found elsewhere we obtain a value of 1,000-2,000. These values compare favourably with those postulated for mouse blood cells. Thus, we propose that telomerase is indeed lacking in somatic tissues. In this regard it is of interest to note that in maize, broken chromosomes are only healed in sporophytic (zygotic) tissues and not in endosperm (terminally differentiated), suggesting that telomerase activity is lacking in the differentiated tissues." [Citations omitted.] PA0 "One alternative explanation for our observations is that in tumours the cells with shorter telomeres have a growth advantage over those with larger telomeres, a situation described for vegetative cells of tetrahymena." [Citations omitted.] PA0 "Telomerase activation may be a late, obligate event in immortalization since many transformed cells and tumour tissues have critically short telomeres. Thus, telomere length and telomerase activity appear to be markers of the replicative history and proliferative potential of cells; the intriguing possibility remains that telomere loss is a genetic time bomb and hence causally involved in cell senescence and immortalization. . . . PA0 Despite apparently stable telomere length in various tumour tissues or transformed cell lines, this length was usually found to be shorter than those of the tissue of origin. These data suggest that telomerase becomes activated as a late event in cell transformation, and that cells could be viable (albeit genetically unstable) with short telomeres stably maintained by telomerase. If telomerase was constitutively present in a small fraction of normal cells, and these were the ones which survived crisis or became transformed, we would expect to find a greater frequency of transformed cells with long telomeres."[Citations omitted.] PA0 "we do not know whether telomere reduction is strictly coupled to cellular proliferation. If the diminution results from incomplete replication of the telomere, such a coupling would be expected; however, other mechanisms, such as exonucleolytic degradation, may operate independent of cell division. In any event, it is clear that the maintenance of telomeres is impaired in somatic cells. An obvious candidate activity that may be reduced or lacking is telomerase. A human telomerase activity that can add TTAGGG repeats to G-rich primers has recently been identified (G. Morin, personal communication). Interestingly, the activity was demonstrated in extracts of HeLa cells, which we found to have exceptionally long telomeres. Other cell types have not been tested yet, but such experiments could now establish whether telomerase activity is (in part) responsible for the dynamics of human chromosome ends." PA0 "Whether long telomeres are a result of selection or simply a neutral change is not clear. Their size seems largely unchanged on passage to subsequent generations, as well as through somatic cell division, so it is unlikely that the extra length is a defence against rapid loss of sequence. Nor are mouse telomeres significantly reduced in size during the animal's lifespan; a 17-month-old individual still showed normal size distribution of fragments characteristic of its strain (data not shown). This, and the much longer telomeres of this short-lived species, suggests that telomere shortening is unlikely to have any causal role in ageing in vivo, in contrast to some recent speculations. The shortening of human telomeres during ageing in vivo may instead indicate that telomere maintenance is another metabolic process that senescent cells are unable to perform as efficiently." PA0 "We propose that during the life span of an organism, telomere shortening does not play a role in the normal aging process. However, mutations or epigenetic changes that affect the activity of the telomerase, like any other genetic change, might affect the life span of the individual in which they occur. . . . PA0 In summary, the telomere shortening with age observed in human diploid fibroblasts may not be a universal phenomenon. Further studies are required to examine telomere length and telomerase activity not only in different cell types as they age but also in the same cell type in different organisms with differing life spans. This would indicate whether telomere shortening plays a causal role in the senescence of a particular cell type or organism." PA0 This model may have direct relevance to tumourigenesis in vivo. For example, the finite lifespan of partially transformed (pre-immortal) cells which lack telomerase might explain the frequent regression of tumours after limited growth in vivo. In bypassing the checkpoint representing normal replicative senescence, transformation may confer an additional 20-40 population doubling during which an additional .apprxeq.2 kbp of telomeric DNA is lost. Since 20-40 doubling (10.sup.6 -10.sup.12 cells in a clonal population) potentially represents a wide range of tumour sizes, it is possible that many benign tumours may lack telomerase and naturally regress when telomeres become critically shortened. We predict that more aggressive, perhaps metastatic tumours would contain immortal cells which express telomerase. To test this hypothesis, we are currently attempting to detect telomerase in a variety of tumour tissues and to correlate activity with proliferative potential. Anti-telomerase drugs or mechanisms to repress telomerase expression could be effective agents against tumours which depend upon the enzyme for maintenance of telomeres and continued cell growth. PA0 "Although it has not been proven that telomere loss contributes to senescence of multicellular organisms, several lines of evidence suggest a causal relationship may exist. . . . PA0 It is also possible that telomere loss with age is significant in humans, but not in mice." [Citations omitted.] PA0 "These and other telomere studies point in a new direction regarding therapeutic targets and strategies to combat cancer. If the cell can heal broken chromosomes preventing genomic disaster, then there may be a way to facilitate or artificially create this process. This could even provide a preventive means of stopping cancer which could be particularly applicable in high risk patients. The difference in telomere length in normal versus tumor cells also suggests a strategy where the loss of telomeres is accelerated. Those cells with the shortest telomeres, such as those of tumor metastasis would be the most susceptible." PA0 "However, such a mechanism is not easily reconciled with the dominance of senescent HDF over young HDF in fusion hybrids, particularly in short-term heterokaryons. One could again invoke the concept of dependence and the RAD9 gene example, such that complete loss of one or a few telomeres leads to the elaboration of a negative signal that prevents initiation of DNA synthesis, thereby mimicking the differentiated state. This idea, although speculative, would not only explain senescent replicative arrest but also the chromosomal aberrations observed in senescent HDS that would specifically ensue after loss of telomeres." [Citations omitted.] PA0 "It has been known for some years that telomeres in human germline cells (e.g. sperm) are longer than those in somatic tissue such as blood. One proposed explanation for this is the absence of telomere repeat addition (i.e. absence of telomerase activity) in somatic cells. If so, incomplete end replication would be expected to result in the progressive loss of terminal repeats as somatic cells undergo successive rounds of division. This is indeed what appears to happen in vivo for humans, with both blood and skin cells showing shorter telomeres with increasing donor age, and telomere loss may contribute to the chromosome aberrations typically seen in senescent cells. Senescence and the measurement of cellular time is an intriguingly complex subject and it will be interesting to see to what extent telomere shortening has a causal role. The large telomeres possessed by both young and old mice would seem to preclude a simple relationship between telomere loss and ageing, but more elaborate schemes cannot be ruled out."[Citations omitted.] PA0 "Telomeric restriction fragments in many transformed cell lines are much shorter than those in somatic cells. In addition, telomere length in tumor tissues is significantly shorter than in the adjacent non-tumor tissue. When transformed cell lines are passaged in vitro there is no change in telomere length. Thus if untransformed cells lack the ability to maintain a telomere length equilibrium, most transformed cells appear to regain it and to reset the equilibrium telomere length to a size shorter than seen in most tissues in vivo. The simplest interpretation of these data is that enzymes, such as telomerase, involved in maintaining telomere length may be required for growth of transformed cells and not required for normal somatic cell viability. This suggests that telomerase may be a good target for anti-tumor drugs." [Citations omitted.] PA0 "The G-rich strand of the telomere is the only essential chromosomal DNA sequence known to be synthesized by the copying of a separate RNA sequence. This unique mode of synthesis, and the special structure and behavior of telomeric DNA, suggest that telomere synthesis could be a target for selective drug action. Because telomerase activity seems to be essential for protozoans or yeast, but not apparently for mammalian somatic cells, I propose that telomerase should be explored as a target for drugs against eukaryotic pathogenic or parasitic microorganisms, such as parasitic protozoans or pathogenic yeasts. A drug that binds telomerase selectively, either through its reverse-transcriptase or DNA substrate-binding properties, should selectively act against prolonged maintenance of the dividing lower eukaryote, but not impair the mammalian host over the short term, because telomerase activity in its somatic cells may normally be low or absent. Obvious classes of drugs to investigate are those directed specifically against reverse transcriptases as opposed to other DNA or RNA polymerases, and drugs that would bind telomeric DNA itself. These could include drugs that selectively bind the G.sup.o G base-paired forms of the G-rich strand protrusions at the chromosome termini, or agents which stabilize an inappropriate GOG base-paired form, preventing it from adopting a structure necessary for proper function in vivo. Telomeres have been described as the Achilles heel of chromosomes: perhaps it is there that drug strategies should now be aimed." [Citations omitted.] PA0 ". . . the work presented in this paper demonstrates that a defect in telomere replication need not result in the death of all cells in a population, suggesting that telomere loss and its relationship to mammalian cellular senescence may have to be examined further."
Shay et al., 27 Experimental Gerontology 477, 1992, and 196 Exp. Cell Res. 33, 1991 describe a two-stage model for human cell mortality to explain the ability of Simian Virus 40 T-antigen to immortalize human cells. The mortality stage 1 mechanism (M1) is the target of certain tumor virus proteins, and an independent mortality stage 2 mechanism (M2) produces crisis and prevents these tumor viruses from directly immortalizing human cells. The authors utilized T-antigen driven by a mouse mammary tumor virus promoter to cause reversible immortalization of cells. The Simian Virus 40 T-antigen is said to extend the replicative life span of human fibroblast by an additional 40-60%. The authors postulate that the M1 mechanism is overcome by T-antigen binding to various cellular proteins, or inducing new activities to repress the M1 mortality mechanism. The M2 mechanism then causes cessation of proliferation, even though the M1 mechanism is blocked. Immortality is achieved only when the M2 mortality mechanism is also disrupted.
It has also been proposed that the finite replicative capacity of cells may reflect the work of a "clock" linked to DNA synthesis in the telomere (end part) of the chromosomes. Olovnikov, 41 J. Theoretical Biology 181, 1973, describes the theory of marginotomy to explain the limitations of cell doubling potential in somatic cells. He states that an:
Harley et al., 345 Nature 458, 1990, state that the amount and length of telomeric DNA in human fibroblasts decreases as a function of serial passage during aging in vitro, and possibly in vivo, but do not know whether this loss of DNA has a causal role in senescence. They also state:
Hastie et al., 346 Nature 866, 1990, while discussing colon tumor cells, state that:
The authors propose that in some tumors telomerase is reactivated, as proposed for HeLa cells in culture, which are known to contain telomerase activity. But, they state:
Harley, 256 Mutation Research 271, 1991, discusses observations allegedly showing that telomeres of human somatic cells act as a mitotic clock shortening with age both in vitro and in vivo in a replication dependent manner. He states:
He proposes a hypothesis for human cell aging and transformation as "[a] semi-quantitative model in which telomeres and telomerase play a causal role in cell senescence and cancer" and proposes a model for this hypothesis.
De Lange et al., 10 Molecular and Cellular Biology 518, 1990, generally discuss the structure of human chromosome ends or telomeres. They state:
Kipling and Cooke, 347 Nature 400, 1990, indicate that mice have large telomeres and discusses this length in relationship to human telomeres. In regard to mice telomers, they state:
D'Mello and Jazwinski, 173 J. Bacteriology 6709, 1991, state:
Hiyama et al., 83 Jpn. J. Cancer Res. 159, 1992, provide findings that "suggest that the reduction of telomeric repeats is related to the proliferative activity of neuroblastoma cells and seems to be a useful indicator of the aggressiveness of neuroblastoma. . . . Although we do not know the mechanism of the reduction and the elongation of telomeric repeats in neuroblastoma, we can at least say that the length of telomeric repeats may be related to the progression and/or regression of neuroblastoma."
Counter et al., 11 EMBO J. 1921, 1992, state "loss of telomeric DNA during cell proliferation may play a role in ageing and cancer." They propose that the expression of telomerase is one of the events required for a cell to acquire immortality and note that:
Levy et al., 225 J. Mol. Biol. 951, 1992, state that:
Windle and McGuire, 33 Proceedings of the American Association for Cancer Research 594, 1992, discuss the role of telomeres and state that:
Goldstein, 249 Science 1129, 1990, discusses various theories of cellular senescence including that of attrition of telomeres. He states:
The role of telomere loss in cancer is further discussed by Jankovic et al. and Hastie et al., both at 350 Nature 1991, in which Jankovic indicates that telomere shortening is unlikely to significantly influence carcinogenesis in men and mice. Hastie et al. agree that if telomere reduction does indeed reflect cell turnover, this phenomenon is unlikely to play a role in pediatric tumors, and those of the central nervous system. Hastie et al., however, feel "our most original and interesting conclusion was that telomere loss may reflect the number of cell division in a tissue history, constituting a type of clock."
Kipling and Cooke, 1 Human Molecular Genetics 3, 1992, state:
Greider, 12 BioEssays 363, 1990, provides a review of the relationship between telomeres, telomerase, and senescence. She indicates that telomerase contains an RNA component which provides a template for telomere repeat synthesis. She notes that an oligonucleotide "which is complementary to the RNA up to and including the CAACCCCAA (SEQ ID NO: 1) sequence, competes with d(TTGGGG)n (SEQ ID NO: 2) primers and inhibits telomerase in vitro" (citing Greider and Blackburn, 337 Nature 331, 1989). She also describes experiments which she believes "provide direct evidence that telomerase is involved in telomere synthesis in vivo." She goes on to state:
Blackburn, 350 Nature 569, 1991, discusses the potential for drug action at telomeres stating:
Lundblad and Blackburn, 73 Cell 347, 1993, discuss alternative pathways for maintainance of yeast telomers, and state that:
Other review articles concerning telomeres include Blackburn and Szostak, 53 Ann. Rev. Biochem. 163, 1984; Blackburn, 350 Nature 569, 1991; Greider, 67 Cell 645, 1991, and Moyzis 265 Scientific American 48, 1991. Relevant articles on various aspects of telomeres include Cooke and Smith, Cold Spring Harbor Symposia on Quantitative Biology Vol. LI, pp. 213-219; Morin, 59 Cell 521, 1989; Blackburn et al., 31 Genome 553, 1989; Szostak, 337 Nature 303, 1989; Gall, 344 Nature 108, 1990; Henderson et al., 29 Biochemistry 732, 1990; Gottschling et al., 63 Cell 751, 1990; Harrington and Grieder, 353 Nature 451, 1991; Muller et al., 67 Cell 815, 1991; Yu and Blackburn, 67 Cell 823, 1991; and Gray et al., 67 Cell 807, 1991. Other articles or discussions of some relevance include Lundblad and Szostak, 57 Cell 633, 1989; and Yu et al., 344 Nature 126, 1990.